The subject matter disclosed herein relates to gas turbine control and, more specifically, to control that accommodates changes in fuel composition.
The stability of power grid frequency requires that the power supplied to the grid equals the power demand from the grid. One of the factors that can cause variation in the power supplied to the grid by gas turbine engines is a change in the composition of the fuel supplied to the gas turbine. Composition variation of the natural gas supply is a common issue. Variation in fuel composition poses concerns for combustion dynamics, combustor blowout, emissions compliance (e.g., NOx, CO), auto-ignition, and flashback. Most premixed combustion systems are designed with sufficient auto-ignition and flashback margin to accommodate expected pipeline compositional variation. Thus, combustion dynamics, emissions compliance, and blowout remain the primary gas turbine operability concerns associated with fuel quality variation.
Most prior approaches to maintain acceptable combustor operability in the face of compositional variation in the fuel have proven costly and slow. For example, because combustor operability is acceptable within a defined range of the Modified Wobbe Index (MWI) [an extension of Wobbe Index (WI), which captures normalized energy output of a given gas, that includes fuel temperature], one approach involves compensating for changes in gas composition with changes in fuel temperature to maintain a constant MWI. This approach involves closed-loop control of MWI by varying the temperature set-point of a gas fuel heater. However, the approach is costly, because it requires a Wobbe meter and/or gas chromatograph, and slow because of the time constant typically associated with heating and cooling large amounts of natural gas.
One form of gas turbine control that is fast enough to accommodate rapid variations in fuel composition involves distribution of the fuel supply among the multiple nozzles of a combustor in a method referred to as combustor fuel staging or fuel split scheduling. When controlled with a model-based algorithm, fuel splits have proven to be a fast effector that accommodates rapid changes in fuel composition. While extremely effective when authority is available, the fuel split control is subject to limits beyond which it is ineffective in maintaining stability. When fuel split is thus limited, it will be unable to accommodate additional rapid changes in fuel composition. Thus, the ability to accommodate rapid fuel variation across the widest possible range of fuels would be appreciated in the power industry.